Secondary battery, battery pack and car

ABSTRACT

A secondary battery includes a positive electrode, a negative electrode containing a metal compound having a lithium ion absorption potential of 0.2V (vs.Li/Li + ) or more, a separator and a nonaqueous electrolyte. The separator is provided between the positive electrode and the negative electrode. The separator comprises cellulose fibers and pores having a specific surface area of 5 to 15 m 2 /g. The separator has a porosity of 55 to 80%, and a pore diameter distribution having a first peak in a pore diameter range of 0.2 μm (inclusive) to 2 μm (exclusive) and a second peak in a pore diameter range of 2 to 30 μm.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2007-249504, filed Sep. 26, 2007,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a secondary battery, a battery packusing the secondary battery and a car using the secondary battery or thebattery pack.

2. Description of the Related Art

Nonaqueous electrolyte batteries using a lithium metal, lithium alloy,lithium compound or carbonaceous materials as the negative electrodeactive material are expected as high energy density batteries andearnest studies are being made as to these nonaqueous electrolytebatteries. Lithium ion batteries comprising a positive electrodecontaining LiCoO₂ or LiMn₂O₄ as an active material and a negativeelectrode containing a carbonaceous material that absorbs and releaselithium ions have been widely put to practical use in portabletelephones so far.

In the case of mounting a battery in vehicles or electric trains, on theother hand, materials superior in chemical or electrochemical stability,strength and corrosion resistance are desired as the structuralmaterials of the positive electrode or negative electrode from theviewpoint of storage characteristics at a high temperature (60° C. ormore), cycle performance and long term reliability of high output.Moreover, high battery performance is required even in cold districts,which means that it is demanded of the battery to have high-outputperformance and long-life performance at a temperature as low as about−40° C. In the meantime, a nonvolatile and inflammable electrolyticsolution is developed from the viewpoint of improving the safetyrequired for a nonaqueous electrolyte. However, this is accompanied byreductions in output performance, low-temperature performance andlong-life performance and therefore, such an electrolytic solution hasnot been put to practical use.

Therefore, from the foregoing descriptions, lithium ion batteries havelarge problems concerning high-temperature durability andlow-temperature output characteristics to mount them in vehicles and thelike. Particularly, it is difficult to mount and use this lithium ionbattery in the engine compartment of a car as a substitute for lead-acidbatteries.

Various attempts have been made to improve negative electrodecharacteristics. JP-A 2002-42889 (KOKAI) discloses that a negativeelectrode having a structure in which a current collector made fromaluminum or an aluminum alloy is made to carry a specified metal, alloyor compound is used in a nonaqueous electrolyte secondary battery.

On the other hand, JP-A 2001-143702 (KOKAI) discloses that primaryparticles of lithium titanate compound represented by the formulaLi_(a)Ti_(3-a)O₄ (0<a<3) and having an average particle diameter lessthan 1 μm are coagulated into granules having an average particlediameter of 5 to 100 μm to form secondary particles, which are used as anegative electrode active material. Also, in JP-A 2001-143702 (KOKAI),there is the description that the coagulation of secondary particles issuppressed by the use of this negative electrode active material, whichincreases the production yield of a negative electrode having a largearea for a large scale battery.

A remarkable attention is focused on lithium iron phosphate(Li_(x)FePO₄) which is a lithium phosphorus compound having an olivinecrystal structure as a positive electrode active material to improve theperformance of the positive electrode and this lithium iron phosphate isexpected to improve thermal stability under high-temperature conditions.On the other hand, studies are made to attain low-temperatureperformance and high-temperature life performance by improving anonaqueous electrolyte.

However, JP-A 11-329395 (KOKAI) discloses that pores of a macroporousmatrix are impregnated with a solution containing a microporous polymerand the resulting macroporous matrix is used as a separator. In theseparator disclosed in JP-A 11-329395 (KOKAI), a macroporous of themacroporous matrix is used to support the microporous polymer and hasalmost no function to support the electrolytic solution. Therefore, theseparator described in JP-A 11-329395 (KOKAI) is inferior in the abilityof impregnating with the electrolytic solution.

On the other hand, in JP-A 5-74437 (KOKAI), there is the descriptionthat a porous film in which a uniform porous coating made of a materialhaving a low hydrogen overvoltage is formed on at least one surfacethereof by deposition is used as separater. There is also thedescription that the surface area of the porous film before the porouscoating is formed by deposition is at least 10 m²/g and the average porediameter of the porous film is about 200 to about 10,000 Å. Theseparator described in JP-A 5-74437 (KOKAI) uses metal such as nickel asthe material having a low hydrogen overvoltage and therefore not onlydevelops short circuits but also is inferior in the ability ofimpregnating with an electrolytic solution.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda secondary battery comprising:

a positive electrode;

a negative electrode containing a metal compound having a lithium ionabsorption potential of 0.2V (vs.Li/Li⁺) or more;

a separator which is provided between the positive electrode and thenegative electrode, comprises cellulose fibers and pores having aspecific surface area of 5 to 15 m²/g, and has a porosity of 55 to 80%and a pore diameter distribution having a first peak in a pore diameterrange of 0.2 μm (inclusive) to 2 μm (exclusive) and a second peak in apore diameter range of 2 to 30 μm; and

a nonaqueous electrolyte.

According to a second aspect of the present invention, there is provideda battery pack comprises a secondary battery, the secondary batterycomprising:

a positive electrode;

a negative electrode containing a metal compound having a lithium ionabsorption potential of 0.2V (vs.Li/Li⁺) or more;

a separator which is provided between the positive electrode and thenegative electrode, comprises cellulose fibers and pores having aspecific surface area of 5 to 15 m²/g, and has a porosity of 55 to 80%and a pore diameter distribution having a first peak in a pore diameterrange of 0.2 μm (inclusive) to 2 μm (exclusive) and a second peak in apore diameter range of 2 to 30 μm; and

a nonaqueous electrolyte.

According to a third aspect of the present invention, there is provideda car comprises a secondary battery, the secondary battery comprising:

a positive electrode;

a negative electrode containing a metal compound having a lithium ionabsorption potential of 0.2V (vs.Li/Li⁺) or more;

a separator which is provided between the positive electrode and thenegative electrode, comprises cellulose fibers and pores having aspecific surface area of 5 to 15 m²/g, and has a porosity of 55 to 80%and a pore diameter distribution having a first peak in a pore diameterrange of 0.2 μm (inclusive) to 2 μm (exclusive) and a second peak in apore diameter range of 2 to 30 μm; and

a nonaqueous electrolyte.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a partially broken sectional view showing a rectangular typesecondary battery according to a first embodiment;

FIG. 2 is a side view of a secondary battery shown in FIG. 1;

FIG. 3 is a perspective view showing a battery module according to asecond embodiment;

FIG. 4 is a characteristic graph showing a discharge curve of a batterypack of Example 1, a battery pack of Comparative Example 1 and a batterypack of lead-acid battery; and

FIG. 5 is a characteristic graph showing the distribution of porediameter of each separator used in Examples 1 to 4 and ComparativeExample 1 when the pore diameter is measured by mercury porosimetry.

DETAILED DESCRIPTION OF THE INVENTION First Embodiment

The inventors have found that when a separator containing cellulosefibers satisfies the following requirements (A) to (C), it hashigh-temperature durability enough to mount a secondary battery and abattery pack comprising the secondary battery in a vehicle and is, atthe same time, superior in output performance at temperatures from roomtemperatures to high temperatures and low-temperature outputperformance.

(A) The porosity of the separator is 55 to 80%.

(B) The separator comprises pores having a specific surface area of 5 to15 m²/g.

(C) The separator has a pore diameter distribution having a first peakat a pore diameter of 0.2 μm (inclusive) to 2 μm (exclusive) and asecond peak at a pore diameter of 2 to 30 μm.

When a secondary battery or a battery pack is mounted in a vehicle orelectric train, it is subjected to a temperature as high as 100° C. ormore depending on where it is placed. For this, a polyolefin separatorwhich is frequently used will be unusable because the resistance of thebattery is remarkably increased by the thermal shrinkage of theseparator. Because the separator containing cellulose fibers has highheat resistance and therefore suppresses thermal shrinkage at a hightemperature of 60° C. or more. Also, if a material containing a metalcompound having a lithium ion absorption potential of 0.2V (vs.Li/Li⁺)or more is used as negative electrode, reduction decomposition of thenonaqueous electrolyte is limited at the high temperature and also, theprecipitation of a metal lithium on the separator is suppressed evenafter a high-temperature cycle or when the battery is overcharged.Moreover, when the battery is stored for a long period of time in acharged state, when the battery is charged by continuous constantvoltage charging called float charging or when the battery isovercharged, the reaction between the negative electrode and theseparator can be limited. In addition to the reduction in products of aside reaction, the separator satisfying the above requirements (A) to(C) can limit the occurrence of the phenomenon that the porosity isreduced by the accumulation of products of a side reaction.

Moreover, the inventors have made it clear that gas generated by theelectrodeposition of water adsorbed to the separator in the productionprocess of the battery causes a deterioration in high-temperaturestorage performance and high-temperature life performance. Also, theinventors have found that when a separator containing cellulose fiberssatisfies the foregoing requirements (A) to (C), the amount of water tobe adsorbed to the separator can be reduced. Because, from the aboveresults, a rise in the internal resistance of the battery and expansion(swelling) of the battery at the high temperature are reduced, asecondary battery or a battery pack can be arranged in the enginecompartment of a car, that is, in the same place as in the case of thelead-acid battery. As a result, the battery according to this embodimentensures that a battery having an outstandingly longer life, smaller-sizeand lighter-weight than the lead-acid battery can be attained.

Also, the inventors have found that the separator satisfying the aboverequirements (A) to (C) and containing cellulose fibers is rapidlyimpregnated even with a nonaqueous electrolyte having high viscositysuch as an ionic liquid, a rise in the internal resistance of thebattery in a long term charge-discharge cycle can be limited and highoutput performance can be accomplished at temperatures from lowtemperatures to high temperatures.

This embodiment enables the use of cellulose fibers that are scarcelyused conventionally as the separator for secondary batteries because ofthe problems concerning a deterioration in life, reliability and safety.

Hereinafter, the separator, positive electrode, negative electrode andnonaqueous electrolyte will be explained.

1) Separator

The separator is a porous body containing cellulose fibers. Examples ofthe porous body may include nonwoven fabrics, films or paper. Also, theporous body can be made substantially of cellulose fibers. Specifically,the ratio of the cellulose fibers in the porous body is desirably 10 to100% by weight. This can improve the heat resistance of the separator.Though the porous body may contain other types of fibers such aspolyolefin fibers, but the ratio of other types of fibers is desirably90% by weight or less (including 0% by weight) so as not to damage theheat resistance.

It is preferable that the fiber diameter of the cellulose fibers be 10μm or less. This improves the affinity of the nonaqueous electrolyte tothe separator and therefore, the resistance of the battery can bereduced. The fiber diameter of the cellulose fibers is more preferably 3μm or less.

The reason why the porosity of the separator is limited to the aboverange will be explained. The separator having a porosity less than 55%is deteriorated in the impregnation with the nonaqueous electrolyte andtherefore, brings about a deterioration in the output performance andlow-temperature output performance of a secondary battery. Also, theaccumulation of the aforementioned products of side reactions causes amore reduction in porosity, giving rise to a reduction in outputperformance along with its use. On the other hand, when the porosityexceeds 80%, the strength of the separator is inferior. The porosity ispreferably in the range of 62 to 80%.

The distribution of pore diameter will be explained. When no peak ispresent at a pore diameter ranging from 0.2 μm (inclusive) to 2 μm(exclusive) even if the porosity is in the above range, the separator isdeteriorated in the impregnation with the nonaqueous electrolyte,leading to a reduction in output performance or low-temperature outputperformance. When no peak is present at a pore diameter ranging from 2μm to 30 μm, the amount of water adsorbed to the separator is increased,leading to a deterioration in high-temperature storage performance. Inmore preferable distribution of pore diameter, a first peak is presentat a pore diameter ranging from 0.3 μm (inclusive) to 2 μm (exclusive)and a second peak is present at a pore diameter ranging from 3 μm to 20μm.

The reason why the pore specific surface area of the separator isdefined in the above range will be explained. When the pore specificsurface area is less than 5 m²/g, the ratio of pores having a diameterrange of 0.2 μm (inclusive) to 2 μm (exclusive) is low and therefore,the nonaqueous electrolyte impregnation ability is deteriorated even ifthe first peak is present, leading to a reduction in output performanceand low-temperature output performance. When the pore specific surfacearea exceeds 15 m²/g, the ratio of pores having a diameter of 2 to 30 μmis low and therefore, the amount of water to be adsorbed to theseparator is increased even if the second peak is present, leading to areduction in high-temperature storage performance. The pore specificsurface area is more preferably in the range of 10 m²/g to 14 m²/g.

The separator preferably has a thickness of 20 to 100 μm and a densityof 0.2 to 0.9 g/cm³. When the thickness and the density are in the aboveranges respectively, the balance between the mechanical strength and areduction in the resistance of the battery can be adjusted, making itpossible to provide a battery which has high output and is resistant tothe development of internal short circuits. Also, a reduction in thermalshrinkage at high temperatures and high-temperature storage performancecan be attained.

The separator used in this embodiment may be obtained in the followingmanner. Specifically, a separator having a pore diameter distributionprovided with a first and second peaks and a porosity ranging of 55 to80% is interposed between the positive electrode and the negativeelectrode, these parts are coiled spirally and the resulting coiled bodyis subjected to a high-temperature and high-pressure press operated at atemperature of 100° C. or more under a pressure of 20 kg/cm² or more.Specifically, the above high-temperature and high-pressure press canreduce the pore specific surface area ranging of 5 to 15 m²/g whilekeeping the porosity and pore diameter distribution within the aboverange. In order to prevent insulation defects, the upper limit of thepressure of the press is preferably designed to be 1000 kg/cm².

2) Positive Electrode

This positive electrode comprises a positive electrode current collectorand a positive electrode layer which is carried on one or both surfacesof the current collector and contains an active material, a conductiveagent and a binder.

Examples of the positive electrode active material include variousoxides and sulfides. Specific examples of the positive electrode activematerial include manganese dioxide such as MnO₂, iron oxide, copperoxide, nickel oxide, lithium-manganese composite oxide, lithium-nickelcomposite oxide such as Li_(x)NiO₂ (⅓≦x≦½), lithium-cobalt compositeoxide such as Li_(x)CoO₂ (⅓≦x≦½), lithium-nickel-cobalt composite oxide,lithium-manganese-cobalt composite oxide, lithium-manganese-nickelcomposite oxide, spinel type lithium-manganese-nickel composite oxidesuch as Li_(x)Mn_(2-y)Ni_(y)O₄ (⅓≦x≦½, 0≦y≦0.5), metal-phosphorous oxidehaving an olivine structure, iron sulfate such as Fe₂(SO₄)₃ and vanadiumoxide such as V₂O₅.

Examples of the lithium-nickel-cobalt composite oxide includeLiNi_(1-y-z)CO_(y)M_(z)O₂ (M represents at least one element selectedfrom the group consisting of Al, Cr and Fe, 0≦y≦0.5, 0≦z≦0.1).

Examples of the lithium-manganese-cobalt composite oxide includeLiMn_(1-y-z)Co_(y)M_(z)O₂ (M represents at least one element selectedfrom the group consisting of Al, Cr and Fe, 0≦y≦0.5, 0≦z≦0.1).

Examples of the lithium-manganese-nickel composite oxide includeLiMn_(x)Ni_(x)M_(1-2x)O₂ (M represents at least one element selectedfrom the group consisting of Co, Cr, Al and Fe, ⅓≦x≦½). Specificexamples of LiMn_(x)Ni_(x)M_(1-2x)O₂ includeLiMn_(1/3)Ni_(1/3)CO_(1/3)O₂ and LiMn_(1/2)Ni_(1/2)CO_(1/2)O₂.

Also, examples of the positive electrode active material also includeorganic materials and inorganic materials. The organic materials includeconductive polymer materials such as a polyaniline and polypyrrole,disulfide type polymer materials. The inorganic materials include sulfur(S) and carbon fluoride. The above x, y and z whose preferable rangesare not described are preferably in the range of 0 to 1. The number oftypes of positive electrode active materials may be designed to be oneor two or more.

Particularly preferable examples of the positive electrode activematerial may include metal-phosphorous oxide having an olivine structureand manganese composite oxide having a spinel structure. This is becausethese positive electrode active materials have the high effect ofsuppressing the oxidation decomposition of the separator.

Examples of the metal-phosphorous oxide having an olivine structure mayinclude Li_(a)M_(b)PO₄ (M represents at least one transition metalelement selected from the group consisting of Mn, Ni, Co and Fe,0≦a≦1.1, 0.8≦b≦1.1) and Li_(a)Fe_(1-c)Mn_(c)PO₄ (0≦a≦1.1, 0≦c≦1). Also,examples of the manganese composite oxide having a spinel structure mayinclude Li_(a)Mn₂O₄ (0≦a≦1).

In particular, iron phosphate represented by Li_(a)FePO₄ (0≦a≦1.1) ispreferable. The above iron phosphate limits the growth of a film formedon the surface of the positive electrode when the battery is stored athigh temperatures and therefore, a rise in the resistance of thepositive electrode during storage is decreased. As a result, the storageperformance of the battery at high temperatures is significantlyimproved and the high-temperature life performance is outstandinglyimproved. If a negative electrode containing a lithium-titanium oxide isused when the iron phosphate is used, the reaction between the electrodeand the nonaqueous electrolyte at high temperatures is limited, leadingto a small rise in the resistance at the boundary of the electrodeduring high-temperature storage.

The particle diameter of primary particles of the positive electrodeactive material is preferably 1 μm or less and more preferably 0.01 to0.5 μm. When the particle diameter is in this range, the influences ofelectronic conducive resistance and the diffusion resistance of lithiumions in the active material are decreased and therefore, the outputperformance is improved. Also, these primary particles may be coagulatedto form secondary particles having a diameter of 10 μm or less.

Carbon particles having an average particle diameter of 0.5 μm or lessare preferably stuck to the surface of the positive electrode activematerial. The amount of these carbon particles to be stuck is preferably0.001 to 3% by weight based on the weight of the positive electrodeactive material. When the amount of the carbon particles to be stuck is0.001% by weight or more, a rise in the resistance of the positiveelectrode can be limited. Also, when the amount of the carbon particlesto be stuck is 3% by weight or less, a rise in the resistance at theboundary between the positive electrode and the nonaqueous electrolytecan be suppressed. As a result, the output performance can be improved.

Examples of the conductive agent include acetylene black, carbon black,graphite and carbon fibers. Particularly, carbon fibers formed by avapor phase growth method and having a fiber diameter of 1 μm or lessare preferable. The use of this carbon fibers ensures that an electronicconductive network inside of the positive electrode is improved so thatthe electronic conductive resistance of the positive electrode isreduced, whereby the output performance of the positive electrode can beimproved.

Examples of the binder include a polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVdF) and fluoro rubber.

As to the ratio of the active material, conductive agent and binder ofthe positive electrode to be compounded, it is preferable that thepositive electrode active material be 80 to 95% by weight, theconductive agent be 3 to 19% by weight and the binder be 1 to 7% byweight.

As the current collector, an aluminum foil or aluminum alloy foil ispreferable and the thickness of the current collector is preferably 20μm or less and more preferably 15 μm or less. The lower limit of thethickness is preferably designed to be 5 μm.

The positive electrode is manufactured, for example, by suspending thepositive electrode active material, conductive agent and binder in aproper solvent and this suspension is applied to the current collector,followed by drying and pressing. The specific surface area of thepositive electrode layer using the BET method is measured in the samemanner as in the case of the negative electrode and is preferably in therange of 0.1 to 2 m²/g.

The positive electrode layer faces the negative electrode layer with theseparator interposed therebetween, and the edge part of the positiveelectrode layer preferably is projected from the edge part of thenegative electrode layer. With this structure, the potential of thepositive electrode layer of the positive electrode edge part can be madeto be equal to that of the positive electrode layer facing the negativeelectrode layer in the center of the positive electrode, whereby thepositive electrode active material at the edge part can be preventedfrom reacting with the nonaqueous electrolyte when the battery isovercharged. When the edge part of the negative electrode layer isprojected over the edge part of the positive electrode layer on thecontrary, the negative electrode active material in the edge partremains unreacted. However, the positive electrode potential at the edgepart is affected by the negative electrode potential of the unreactededge part. As a result, the positive electrode potential at the edgepart reaches an overcharge potential when the battery is fully charged,and there is therefore the possibility of deteriorated life performance.Therefore, it is desirable to coil the positive electrode layer and thenegative electrode layer in such a manner that the area of the positiveelectrode layer is larger than that of the negative electrode layer andthe positive electrode layer is projected over the negative electrodelayer in the condition that both layers are facing each other, tothereby constitute an electrode group.

The ratio (Sp/Sn) of the area (Sp) of the positive electrode layer tothe area (Sn) of the negative electrode layer is preferably in the rangeof 0.85 to 0.999 and more preferably in the range of 0.95 to 0.99. Whenthe ratio is made to be in the above range, the amount of gas generatedfrom the negative electrode can be decreased when the battery is storedat high temperatures in a charged state and when the battery isfloat-charged (continuous constant voltage charged) at high temperaturesand it is therefore possible to improve the storage performance. Whenthe ratio is less than 0.85, there is a fear that the capacity of thebattery is decreased. Also, at this time, the ratio (Lp/Ln) of the width(Lp) of the positive electrode to the width (Ln) of the negativeelectrode is preferably in the range of 0.9 to 0.99.

3) Negative Electrode

This negative electrode comprises a negative electrode current collectorand a negative electrode layer which is carried on one or both surfaceof the current collector and contains an active material, a conductiveagent and a binder.

As the negative electrode material, a metal compound having a lithiumion absorption potential of 0.2V (vs.Li/Li⁺) or more is used. The reasonwhy the lithium ion absorption potential is defined in the above rangewill be explained. Examples of the active material that absorbs lithiumions at a potential less than 0.2V (vs.Li/Li⁺) include carbonaceousmaterials and lithium metal. A negative electrode containing an activematerial that absorbs lithium ions at a potential less than 0.2V(vs.Li/Li⁺) undergoes a reduction decomposition with a nonaqueouselectrolyte at high temperatures and also, precipitates metal lithium ina long term cycle. For this, in the separator according to thisembodiment, internal short circuits are easily developed. As a result,not only the output performance and charge-discharge cycle performancebut also the entire battery performance is deteriorated. The lithium ionabsorption potential is more preferably 0.4V (vs.Li/Li⁺) or more and theupper limit of the lithium ion absorption potential is preferably 3V(vs.Li/Li⁺) and more preferably 2V (vs.Li/Li⁺).

The negative electrode active material of a metal compound capable ofabsorbing lithium ions at a potential range of 0.2 to 3V (vs.Li/Li⁺) ispreferably a metal oxide, metal sulfide or metal nitride.

Examples of the metal oxide include Li_(x)TiO₂ (0≦x, more preferably0≦x≦1), a titanium type oxide, lithium-titanium oxide, tungsten oxide(for example, WO₃), amorphous tin oxide (for example,SnB_(0.4)PO_(0.6)O_(3.1)), tin-silicon oxide (for example, SnSiO₃) andsilicon oxide (SiO).

Examples of the titanium type oxide include TiO₂. As the crystalstructure of TiO₂, an anatase type or B (bronze) type is preferable andtitanium type oxides which are heat-treated at 300 to 600° C. and haveless crystallinity are preferable. Other examples of the titanium typeoxide may include composite oxides containing Ti and at least oneelement selected from the group consisting of P, V, Sn, Cu, Ni, Mn andFe. Examples of the composite oxides may include TiO₂—P₂O₅, TiO₂—V₂O₅,TiO₂—P₂O₅—SnO₂ and TiO₂—P₂O₅-MeO (Me is at least one element selectedfrom the group consisting of Cu, Ni and Fe). The above composite oxidespreferably have less crystallinity and preferably have a microstructurein which a crystal phase and an amorphous phase coexist or an amorphousphase exists independently. Such a microstructure may improve cycleperformance.

Examples of the lithium-titanium oxide may include Li_(x)TiO₂ (0<x, morepreferably 0<x≦1), those having a spinel structure (for example,Li_(4+x)Ti₅O₁₂ (−1≦x≦3)), those having a rhamsdelite structure (forexample, Li_(2+x)Ti₃O₇ (−1≦x≦3), Li_(1+x)Ti₂O₄ (0≦x, more preferably0≦x≦1), Li_(1.1+x)Ti_(1.8)O₄ (0≦x, more preferably 0≦x≦1) andLi_(1.07+x)Ti_(1.86)O₄ (0≦x, more preferably 0≦x≦1). More preferableexamples of the lithium-titanium oxide may include Li_(x)TiO₂ (0≦x, morepreferably 0<x≦1), those having a spinel structure and those having arhamsdelite structure. Among these compounds, having a rhamsdelitestructure, Li_(1.1+x)Ti_(1.8)O₄ (0≦x, more preferably 0≦x≦1) ispreferable.

Examples of the metal sulfide include titanium sulfide (for example,TiS₂), molybdenum sulfide (for example, MOS₂), iron sulfide (forexample, FeS, FeS₂ and Li_(x)FeS₂).

Examples of the metal nitride include lithium-cobalt nitride (forexample, Li_(x)Co_(y)N, 0<x<4, 0<y<0.5).

The average particle diameter of primary particles of the negativeelectrode active material is preferably in the range of 0.001 to 1 μm.Also, the particle shape may be any shape including a granular shape andfiber shape to obtain a good performance. When the particle has a fibershape, the fiber diameter is preferably 0.1 μm or less.

The average particle diameter of the negative electrode active materialis preferably 1 μm or less and the specific surface area of the negativeelectrode active material is in the range of 3 to 200 m²/g when measuredby the BET method using N₂ adsorption. This structure allows thenegative electrode to have stronger affinity to the nonaqueouselectrolyte.

The specific surface area of the negative electrode may be designed tobe in the range of 3 to 50 m²/g. The specific surface area is morepreferably in the range of 5 to 50 m²/g.

The porosity of the negative electrode excluding the current collectoris preferably in the range of 20 to 50%. This ensures the production ofa negative electrode having high affinity to the nonaqueous electrolyteand a high density. The porosity is more preferably 25 to 40%.

The negative electrode current collector is preferably made of analuminum foil or aluminum alloy foil. When this aluminum foil oraluminum alloy foil is used for the negative electrode currentcollector, a deterioration during storage caused by overdischarge athigh temperatures can be prevented.

The thickness of the aluminum foil or aluminum alloy foil is preferably20 μm or less and more preferably 15 μm or less. The lower limit of thethickness is preferably designed to be 5 μm. The purity of the aluminumfoil is preferably 99.99 wt % or more. As the aluminum alloy, alloyscontaining magnesium, zinc or silicon are preferable. On the other hand,the content of transition metals such as iron, copper, nickel andchromium is preferably 100 wt-ppm or less.

Examples of the conductive agent may include acetylene black, carbonblack, cokes, carbon fibers, graphite, metal compound powder and metalpowder. More preferable examples of the conductive agent include cokeswhich are heat-treated at 800 to 2000° C. and have an average particlediameter of 10 μm or less, graphite, TiO, TiC, TiN and metal powderssuch as Al, Ni, Cu and Fe.

Examples of the binder include a polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVdF), fluoro rubber, styrene-butadiene rubberand core-shell binder.

As to the ratio of the active material, conductive agent and binder ofthe negative electrode, it is preferable that the negative electrodeactive material is 80 to 95% by weight, the conductive agent is 1 to 18%by weight and the binder is 2 to 7% by weight.

The negative electrode is manufactured by suspending the aforementionednegative electrode active material, conductive agent and binder in aproper solvent and this suspension is applied to the current collector,drying and heat pressing.

4) Nonaqueous Electrolyte

Examples of the nonaqueous electrolyte include a liquid organicelectrolyte prepared by dissolving an electrolyte in an organic solvent,gel-like organic electrolyte obtained by making a composite of anorganic solvent and a polymer material and solid nonaqueous electrolyteobtained by making a composite of a lithium salt electrolyte and apolymer material. Also, an ionic liquid (ionic molten material)containing lithium ion may be used as the nonaqueous electrolyte.Examples of the polymer material may include a polyvinylidene fluoride(PVdF), polyacrylonitrile (PAN) and polyethylene oxide (PEO).

Particularly, it is preferable to use an organic electrolyte having aboiling point of 200° C. or more or an ionic liquid. The organicelectrolyte having a boiling point of 200° C. or more or ionic liquidare reduced in vapor pressure and in the generation of gas andtherefore, excellent durability and life performance can be obtained ina high temperature environment at 80° C. or more when a secondarybattery is used in vehicles or the like.

The liquid organic electrolyte is prepared by dissolving an electrolytein a concentration of 0.5 to 2.5 mol/L in an organic solvent.

Examples of the electrolyte include LIBF₄, LiPF₆, LiAsF₆, LiClO₄,LiCF₃SO₃, LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, Li(CF₃SO₂)₃C and LiB[(OCO)₂]₂.The number of types of electrolyte to be used can be one or two or more.Among these compounds, a compound containing lithium tetrafluoroborate(LiBF₄) is preferable. This improves the chemical stability of theorganic solvent and the resistance of the film on the negative electrodecan be reduced, resulting in a significant improvement inlow-temperature performance and cycle life performance.

Examples of the organic solvent may include cyclic carbonates such aspropylene carbonate (PC) and ethylene carbonate (EC), chain carbonatessuch as diethyl carbonate (DEC) and dimethyl carbonate (DMC) and methylethyl carbonate (MEC), chain ethers such as dimethoxyethane (DME) anddiethoethane (DEE), cyclic ethers such as tetrahydrofuran (THF) anddioxolan (DOX), γ-butyrolactone (GBL), acetonitrile (AN) and sulfolane(SL). These organic solvents may be used either singly or incombinations of two or more. If, for example, propylene carbonate (PC),ethylene carbonate (EC) or γ-butyrolactone (GBL) is used as a primarycomponent, this is preferable because the boiling point is 200° C. ormore and the solvent has high thermal stability. In the case ofcontaining γ-butyrolactone (GBL), the output performance at lowtemperatures is improved, which is desirable. Also, because ahigh-concentration lithium salt can be used, the concentration of thelithium salt in the organic solvent can be in the range of 1.5 to 2.5mol/L. When the concentration of the lithium salt is made to be 1.5mol/L or more, a drop in lithium ion concentration at the boundarybetween the positive electrode and the nonaqueous electrolyte during thecourse of discharging under a large current can be reduced. Also, whenthe concentration of lithium salt is made to be 2.5 mol/L or less, arise in the viscosity of the nonaqueous electrolyte can be limited andtherefore, the transfer speed of lithium ions can be improved.Accordingly, high output can be achieved even at low temperatures.

The electrolyte containing an ionic liquid will be explained.

The ionic liquid means a salt in which at least a part thereof exhibitsa liquid state at normal temperature. Here, the normal temperature meansa temperature range in which a power source works in usual. Thetemperature range in which a power source usually works means that theupper limit of the temperature is about 120° C. (about 60° C. dependingon the case) and the lower limit is about −40° C. (−20° C. depending onthe case). Particularly, the temperature range of −20 to 60° C. isappropriate.

As the ionic liquid containing lithium ions, an ionic liquid containinglithium ions, an organic cation and an anion is preferably used. Also,this ionic liquid is preferably in a liquid state at the ambienttemperature or lower.

Examples of the above organic cation include alkylimidazolium ions andquaternary ammonium ions having a skeleton represented by the followingformula (1).

As the above alkylimidazolium ion, a dialkylimidazolium ion, atrialkylimidazolium ion and a tetraalkylimidazolium ion and the like arepreferable.

As the dialkylimidazolium ion, 1-methyl-3-ethylimidazolium ion (MEI⁺) ispreferable. As the trialkylimidazolium ion,1,2-diethyl-3-propylimidazolium ion (DMPI⁺) is preferable. As thetetraalkylimidazolium ion, 1,2-diethyl-3,4(5)-dimethylimidazolium ion ispreferable.

As the above quaternary ammonium ion, a tetraalkylammonium ion, a cyclicammonium ion and the like are preferable. As the tetraalkylammonium ion,dimethylethylmethoxyethylammonium ion,dimethylethylmethoxymethylammonium ion, dimethylethylethoxyethylammoniumion and trimethylpropylammonium ion are preferable.

When the above alkylimidazolium ion or a quaternary ammonium ion(especially, a tetraalkylammonium ion) is used, the melting point ispreferably 100° C. or less and more preferably 20° C. or less. Also, thereactivity with the negative electrode can be reduced.

The concentration of the above lithium ion is preferably 20 mol % orless. The concentration is more preferably in the range of 1 mol % to 10mol %. When the concentration is made to be in the above range, an ionicliquid can stability exist at a temperature as low as 20° C. or less.Also, the viscosity can be lowered at temperatures equal to or lowerthan normal temperature and the ion conductivity can be heightened.

The above anion preferably is one or more anions selected from the groupof, for example, BF₄ ⁻, PF₆ ⁻, AsF₆ ⁻, ClO₄ ⁻, CF₃SO₃ ⁻, CF₃COO⁻,CH₃COO⁻, CO₃ ²⁻, (FSO₂)₂N⁻, N(CF₃SO₂)₂ ⁻, N(C₂F₅SO₂)₂ ⁻ and (CF₃SO₂)₃C⁻.The coexistence with plural anions makes it possible to form an ionicliquid having a melting point of 20° C. or less. More preferableexamples of the anion include BF₄ ⁻, (FSO₂)₂N⁻, CF₃SO₃ ⁻, CF₃COO⁻,CH₃COO⁻, CO₃ ²⁻, N(CF₃SO₂)₂ ^(—), N(C₂F₅SO₂)₂ ⁻ and (CF₃SO₂)₃C⁻. Anionic liquid having a melting point of 0° C. or less is formed moreeasily by these anions.

5) Container

As the container that receives the positive electrode, negativeelectrode and nonaqueous electrolyte, a metal container or a laminatefilm container may be used.

As the metal container, a metallic can which is made of aluminum, analuminum alloy, iron or stainless and has a rectangular or cylindricalshape may be used. The plate thickness of the container is preferably0.5 mm or less and more preferably 0.3 mm or less.

Examples of the laminate film may include a multilayer film obtained bycoating an aluminum foil with a resin film. As the resin, polymers suchas a polypropylene (PP), polyethylene (PE), nylon, polyethyleneterephthalate (PET) may be used. Also, the thickness of the laminatefilm is preferably 0.2 mm or less and also, the purity of the aluminumfoil is preferably 99.5% or more.

The metallic can made of an aluminum alloy is preferably constituted ofan alloy containing an element such as manganese, magnesium, zinc andsilicon and having an aluminum purity of 99.8% or less. An outstandingincrease in the strength of the metallic can made of an aluminum alloyoffers possibility of a reduction in the wall thickness of the can. As aresult, a thin, light-weight and high-output battery superior in heatradiation ability can be attained.

A rectangular type secondary battery according to this embodiment isshown in FIGS. 1 and 2. As shown in FIG. 1, an electrode group 1 isreceived in a rectangular cylindrical metal container 2. The electrodegroup 1 has a structure in which a positive electrode 3 and a negativeelectrode 4 are coiled spirally with a separator 5 interposedtherebetween into a flat form. The nonaqueous electrolyte (not shown) issupported by the electrode group 1. As shown in FIG. 2, band-shapedpositive electrode leads 6 are each connected electrically to pluralpoints of the positive electrode 3 positioned at the end surface of theelectrode group 1. Also, band-shaped negative electrode leads 7 are eachconnected electrically to plural points of the negative electrode 4positioned at the end surface of the electrode group 1. These pluralpositive electrode leads 6 are electrically connected to a positiveelectrode electroconductive tab (p-tab) 8 in a bundled state. A positiveelectrode terminal is constituted of the positive electrode lead 6 andthe p-tab 8. The negative electrode leads 7 are electrically connectedto a negative electrode electroconductive tab (n-tab) 9 in a bundledstate. A negative electrode terminal is constituted of the negativeelectrode lead 7 and the n-tab 9. A metal seal plate 10 is secured to anopening part of the metal container 2 by welding. The p-tab 8 and then-tab 9 are respectively drawn externally from drawing hole formed inthe seal plate 10. The inside peripheral surface of each drawing hole ofthe seal plate 10 is coated with an insulation member 11 to avoid shortcircuits developed by the contact with the p-tab 8 and with the n-tab 9.

Second Embodiment

A battery pack according to a second embodiment comprises a batteryassembly comprising the secondary batteries according to the firstembodiment. Though these secondary batteries may be connected in seriesor in parallel, it is particularly preferable that these secondarybatteries be connected in series and n units (n is 1 or more) eachconstituted of six secondary batteries be connected in series. Ametal-phosphorous oxide having an olivine structure (for example, ironphosphate represented by Li_(a)FePO₄ (0≦a≦1.1)) is used as the positiveelectrode active material and also, Li_(x)TiO₂ (0≦x, more preferably0≦x≦1) or a lithium-titanium oxide having a rhamsdelite structure isused as the negative electrode active material, whereby the averagevoltage of the battery is 2V. In this case, when the number of batteriesis made to be n times (n is 1 or more) the number of series of sixbatteries, the voltage of one unit constituted of 6 batteries connectedin series is 12V, which is very improved in exchangeability for alead-acid battery pack. When Li_(x)TiO₂ (0≦x, more preferably 0≦x≦1) ora lithium-titanium oxide having a rhamsdelite structure is used as thenegative electrode active material, the voltage curve can be made tohave a moderate gradient and therefore, the charged condition of thebattery can be easily seen only by monitoring the voltage in the samemanner as in the case of a lead-acid battery. As a result, in thebattery pack comprising n units each constituted of 6 batteriesconnected in series, the influence of a dispersion between batteries isdecreased, making it possible to control the battery pack only bymonitoring the voltage.

An embodiment of a battery assembly to be used in the battery pack isshown in FIG. 3. A battery assembly 21 shown in FIG. 3 comprisesplurality of rectangular secondary batteries 22 ₁ to 22 ₅ according tothe first embodiment. The p-tab 8 of the secondary battery 22 ₁ iselectrically connected with the n-tab 9 of the secondary battery 22 ₂disposed adjacent to the secondary battery 22 ₁ by a lead 23. Also, thep-tab 8 of the secondary battery 22 ₂ is electrically connected with then-tab 9 of the secondary battery 22 ₃ disposed adjacent to the secondarybattery 22 ₂ by a lead 23. The secondary batteries 22 ₁ to 22 ₅ areconnected in series in this manner.

The present invention will be explained by way of examples, which are,however, not intended to be limiting of the invention.

EXAMPLE 1

As the positive electrode active material, LiFePO₄ particles having anolivine structure were used in which 0.1% by weight of carbonmicroparticles (average particle diameter: 0.005 μm) were stuck to thesurface thereof. The average particle diameter of primary particles ofLiFePO₄ was 0.1 μm. To the LiFePO₄ particles were added vapor phasegrowth carbon fibers having a fiber diameter of 0.1 μm as a conductiveagent in an amount of 3% by weight, graphite powder in an amount of 5%by weight and PVdF as a binder in an amount of 5% by weight based on thetotal weight of the positive electrode. These components were dispersedin n-methylpyrrolidone (NMP) to prepare a slurry. Then, the obtainedslurry was applied to both surfaces of an aluminum alloy foil (purity:99%) 15 μm in thickness, dried and then subjected to a pressing step tomanufacture a positive electrode. In the obtained positive electrode,the density of the electrode was 2.2 g/cm³, the thickness of thepositive electrode layer on one surface of the current collector was 43μm and the specific surface area of the positive electrode layer was 5m²/g.

Also, as the negative electrode active material, a lithium titanatepowder was prepared which having an average primary particle diameter of0.3 μm, a BET specific surface area of 15 m²/g, a lithium ion absorptionpotential of 1.50 V (vs.Li/Li⁺) and having a rhamsdelite structure andis represented by the formula Li₂Ti₃O₇. This negative electrode activematerial, a graphite power having an average particle diameter of 6 μmas an electroconductive agent and PVdF as a binder were compounded in aratio by weight of 95:3:2. Then, the mixture was dispersed in ann-methylpyrrolidone (NMP) solvent and then stirred with ball mill at1000 rpm for 2 hours to prepare a slurry. Then, the obtained slurry wasapplied to an aluminum alloy foil (purity: 99.3%) 15 μm in thickness,dried and then subjected to a heat pressing step to manufacture anegative electrode. In the obtained negative electrode, the density ofthe electrode was 2.2 g/cm³, the thickness of the negative electrodelayer on one surface of the current collector was 59 μm and the porosityof the negative electrode excluding the current collector was 35%. TheBET specific surface area of the negative electrode layer (BET surfacearea per 1 g of the negative electrode layer) was 10 m²/g.

A method of measuring particles of the negative electrode activematerial will be shown below.

Specifically, about 0.1 g of a sample, a surfactant, and 1 to 2 mL of adistilled water were put in a beaker, and the distilled water wassufficiently stirred, followed by pouring the stirred system in astirring water vessel. Under this condition, the light intensitydistribution was measured every 2 seconds and measured 64 times in totalby using SALD-300, which is a Laser Diffraction Particle Size Analyzermanufactured by Shimadzu Corporation, to analyze the particle diameterdistribution data.

The BET specific surface areas of the negative electrode active materialand negative electrode were measured using N₂ adsorption in thefollowing condition.

When the BET specific surface area of the negative electrode activematerial was measured, 1 g of a powdery negative electrode activematerial was used as a sample. When the BET specific surface area of thenegative electrode was measured, on the other hand, two pieces ofnegative electrode 2×2 cm² in size were cut off as samples. As the BETspecific surface area measuring apparatus, one manufacture by YuasaIonics Inc. was used and in this case, nitrogen gas was used as theadsorbing gas.

The porosity of the negative electrode was calculated on the basis ofthe difference in volume between the negative electrode layer that wastested and the negative electrode layer at the time when the porositywas 0%. In this calculation, the difference noted above was regarded asthe pore volume. Incidentally, where the negative electrode layers wereformed on both surfaces of the current collector, the volume of thenegative electrode layer used for the calculation noted above representsthe sum of the volumes of the negative electrode layers on both surfacesof the current collector.

On the other hand, a 30-μm-thick nonwoven fabric made from regeneratedcellulose fibers using pulp as starting material and having an averagefiber diameter of 0.3 μm was prepared as a separator. The separator wasmade to be in contact with the positive electrode and to cover thepositive electrode and the negative electrode was made to face thepositive electrode with the separator disposed therebetween. Then, thesemembers were coiled spirally. The obtained coiled body was subjected toa high-temperature and high pressure press operated at 120° C. under apressure of 25 kg/cm² to form a flat-shaped electrode group. Theelectrode width (Lp) of the positive electrode layer at this time was 50mm, the electrode width (Ln) of the negative electrode layer at thistime was 51 mm, and therefore, Lp/Ln was 0.98. The ratio (Sp/Sn) of thearea of the positive electrode layer to the area of the negativeelectrode layer was 0.98. The porosity, position of a peak of porediameter distribution and pore specific surface area before and afterthe separator was pressed are shown in Table 1.

Moreover, this electrode group was received in a thin type metallic canmade from an aluminum alloy (Al purity: 99%) 0.5 mm in thickness.

In the meantime, a solvent in which propylene carbonate (PC),γ-butyrolactone (BL) and ethylene carbonate (EC) were mixed in a ratioby volume of 30:40:30 was prepared as an organic solvent. 2.0 mol/L oflithium tetrafluoroborate (LiBF₄) as lithium salt was dissolved in thisorganic solvent to prepare a liquid organic electrolyte (nonaqueouselectrolytic solution). The boiling point of the obtained organicelectrolyte was 220° C. This organic electrolyte was injected into theelectrode group in the container to manufacture a rectangular-typenonaqueous electrolyte secondary battery having a thickness of 16 mm, awidth of 40 mm and a height of 60 mm and having the structure shown inFIG. 1.

EXAMPLES 2 TO 10 AND COMPARATIVE EXAMPLES 1 TO 5

Thin type secondary batteries were manufactured in the same manner as inthe above Example 1 except that the separator, positive electrode activematerial, negative electrode active material, organic solvent or organicelectrolyte, press temperature and press pressure as shown in Table 1below were used.

With regard to the obtained secondary batteries, the discharge capacityof each battery was measured when the battery was charged at 25° C.under a constant current of 6 A to 2.8V for 6 minutes and then,discharged to 1.5V under a current of 3 A. Also, the secondary batterywas fully charged up to peak voltage where it was in 100% charged stateat 25° C. and then, stored at 70° C. for 3 months to measure theresidual capacity and recovery capacity of the battery at 25° C.,thereby finding the high-temperature storage performance of the battery.Moreover, after the secondary battery was charged until the charge ratiobecame 50%, it was made to output at 25° C. for 10 seconds to measure amaximum output as the output performance at 25° C. Also, after thesecondary battery was charged until the charge ratio became 50%, it wasmade to output at −30° C. for 10 seconds to measure a maximum output asthe output performance at −30° C.

Also, only the separator used in each secondary battery was dried at 80°C. for 24 hours and stored in the atmosphere of a temperature of 25° C.and a humidity of 20% for 5 hours, to measure the amount of water in theseparator. The measurement of the amount of water was made using aKarl-Fisher moisture content measuring device in such low-humidityconditions that the dew point was −40° C. or less.

The results of measurement are shown in Table 3.

TABLE 1 Before high-pressure pressing After high-pressure pressing Porespecific Pore specific Porosity Pore diameter surface area Porosity Porediameter surface area Material (%) peak position (m²/g) (%) peakposition (m²/g) Example 1 Cellulose 70 1 μm, 12 μm 16 65 1 μm, 9 μm 12Example 2 Cellulose 70 1 μm, 12 μm 16 65 1 μm, 9 μm 12 Example 3Cellulose 70 1 μm, 12 μm 16 65 1 μm, 9 μm 12 Example 4 Cellulose 70 1μm, 12 μm 16 65 1 μm, 9 μm 12 Example 5 Cellulose 70 1 μm, 12 μm 16 65 1μm, 9 μm 12 Example 6 Cellulose 70 1 μm, 12 μm 16 65 1 μm, 9 μm 12Example 7 Cellulose 60 0.3 μm, 8 μm   20 55 0.2 μm, 5 μm   15 Example 8Cellulose 75 1 μm, 20 μm 20 80 1.8 μm, 15 μm  5 Example 9 Cellulose 75 1μm, 18 μm 20 70 1.8 μm, 12 μm  10 Example 10 Cellulose 70 1 μm, 12 μm 1665 1 μm, 9 μm 12 Comparative Polyethylene 42 0.2 μm 52 40 0.2 μm 50Example 1 film Comparative Polyethylene 42 0.2 μm 52 40 0.2 μm 50Example 2 film Comparative Polypropylene 47 0.2 μm 62 45 0.2 μm 60Example 3 Comparative Cellulose 47 0.2 μm, 2 μm   22 45 0.3 μm, 2 μm  20 Example 4 Comparative Cellulose 90 5 μm, 20 μm 62 82  5 μm, 32 μm 40Example 5

TABLE 2 Negative electrode Organic Press Press active material, solventtemperature pressure Positive electrode Negative electrode lithium ionabsorption or organic (° C.) (kg/cm²) active material active materialpotential (vs.Li/Li⁺) electrolyte Example 1 120 25 LiFePO₄ Li₂Ti₃O₇ 1.50V EC/GBL/PC Example 2 120 25 LiFePO₄ Li₄Ti₅O₁₂ 1.55 V EC/GBL/PC Example3 120 25 LiCoO₂ Li₄Ti₅O₁₂ 1.55 V EC/GBL/PC Example 4 120 25 LiFePO₄ TiO₂(B) 1.50 V EC/GBL/PC Example 5 120 25 LiNi₁/₃Mn₁/₃Co₁/₃O₂ Li₄Ti₅O₁₂ 1.55V EC/GBL/PC Example 6 120 25 LiFePO₄ Li₄Ti₅O₁₂ 1.55 V MEI⁺/BF₄ ⁻ (Ionicliquid) Example 7 150 25 LiFePO₄ Li₄Ti₅O₁₂ 1.55 V EC/GBL/PC Example 8150 25 LiFePO₄ Li₄Ti₅O₁₂ 1.55 V EC/GBL/PC Example 9 100 25 LiFePO₄Li₄Ti₅O₁₂ 1.55 V EC/GBL/PC Example 10 120 25 LiMn₂O₄ Li₄Ti₅O₁₂ 1.55 VEC/PC(1:2) Comparative 80 10 LiCoO₂ Graphite  0.2 V EC/GBL/PC Example 1Comparative 80 10 LiFePO₄ Li₄Ti₅O₁₂ 1.55 V EC/GBL/PC Example 2Comparative 100 25 LiNi₁/₃Mn₁/₃Co₁/₃O₂ Graphite  0.2 V EC/GBL/PC Example3 Comparative 120 25 LiFePO₄ Li₄Ti₅O₁₂ 1.55 V EC/GBL/PC Example 4Comparative 120 1 LiFePO₄ Li₄Ti₅O₁₂ 1.55 V EC/GBL/PC Example 5

TABLE 3 70° C. storage 70° C. storage Separator, three-month three-monthOutput Low-temperature water content 25° C. discharge residual capacityrecovery capacity at 25° C. output at −30° C. (ppm) capacity (mAh) rate(%) rate (%) (W) (W) Example 1 500 3300 70 90 100 15 Example 2 500 330070 90 150 18 Example 3 500 3300 65 85 250 30 Example 4 500 3300 70 90150 15 Example 5 500 3300 73 92 190 19 Example 6 500 3300 73 92 100 5Example 7 700 3500 75 90 150 10 Example 8 300 3300 60 80 160 20 Example9 400 3100 72 90 150 10 Example 10 500 3100 70 60 200 5 Comparative 103500 10 20 120 10 Example 1 Comparative 10 3300 50 60 50 5 Example 2Comparative 20 3300 20 30 40 4 Example 3 Comparative 1000 3300 70 70 405 Example 4 Comparative 2000 3300 30 70 80 10 Example 5

As is clear from Tables 1 to 3, the batteries of Examples 1 to 10 aresuperior to Comparative Examples 1 to 5 in the residual capacity ratioand recovery capacity after stored at 70° C., that is, life performanceat 70° C. and output performance at 25° C. and −30° C.

On the other hand, in Comparative Examples 1 to 3 using a polyolefinseparator having the characteristics that no second peak was present inthe pore diameter distribution, the porosity was less than 55% and porespecific surface area exceeded 15 m²/g, each of Comparative Examples wasinferior in either the storage performance at 70° C. or outputperformance. Also, in Comparative Example 4 using a cellulose separatorhaving a porosity of less than 55% and a pore specific surface areaexceeding 15 m²/g, the battery was inferior in output performance at 25°C. On the other hand, in Comparative Example 5 using a celluloseseparator having the characteristics that no first peak was present inthe pore diameter distribution, the porosity exceeding 80% and a porespecific surface area exceeding 15 m²/g, the residual capacity ratio andoutput performance at 25° C. were low.

Also, as to the content of water in the separator, the content of waterwas more increased in the separators of Comparative Examples 4 and 5than in the separators of Examples 1 to 10, and it was thereforeconfirmed that the use of the separator which easily adsorbs moisturebrings about a deterioration in the performance of the battery.

Moreover, six secondary batteries of Example 1 were connected in seriesto constitute a battery assembly, thereby obtaining a battery pack ofExample 1. Also, three secondary batteries of Comparative Example 1 wereconnected in series to constitute a battery assembly, thereby obtaininga battery pack of Comparative Example 1. FIG. 4 is a discharge curve in0.1 C discharge. The discharge curve of the battery pack of Example 1was similar to the discharge curve of a 12V type lead-acid battery packand it was therefore confirmed that the battery pack of Example 1 wassuperior in exchangeability for a lead-acid battery.

Also, the battery pack of Example 1 was subjected to a storage testcarried out at 70° C. for 3 months in the same condition as above. As aresult, the residual capacity and recovery capacity were 70% and 88%respectively. Also, the battery pack was exchanged for a lead-acidbattery pack placed in the engine compartment of a car having adisplacement of 1500 cc to make an engine starting test and as a result,the engine could be started in conditions ranging from winter at anexternal temperature of about −10° C. to summer at an externaltemperature of about 35° C.

With regard to the separator, the porosity, the peak position in thepore diameter distribution and pore specific surface area were foundfrom the pore diameter distribution measured by the mercury porosimetry.The method of measuring the pore diameter distribution will beexplained.

The pore diameter distribution of the separator before thehigh-temperature and high-pressure pressing, that is, before theseparator was incorporated into the battery was measured in thefollowing manner. Specifically, the separator was cut into a size of25×75 mm and the obtained sample was set to Automatic Porosimeter Autopore IV9500 (trade name, manufactured by Shimazu) to measure the porediameter distribution at 25° C. by the mercury porosimetry.

The pore diameter distribution of the separator after thehigh-temperature and high-pressure pressing, that is, after theseparator was incorporated into the battery was measured in thefollowing manner. Specifically, the battery was decomposed in suchlow-humidity conditions that the dew point was −40° C. or less, to takeout the separator, which was then cut into a size of 25×75 mm. Theobtained sample was sufficiently washed by dimethyl carbonate todissolve the nonaqueous electrolyte stuck to the sample in dimethylcarbonate and then dried under vacuum at 80° C. for 12 hours to removethe nonaqueous electrolyte completely. This sample was set to AutomaticPorosimeter Auto pore IV9500 (trade name, manufactured by Shimazu) tomeasure the pore diameter distribution at 25° C. by the mercuryporosimetry.

With regard to the separators to be used in Examples 1, 2, 3 and 4 andComparative Example 1, the pore diameter distribution after thehigh-temperature and high-pressure pressing is shown in FIG. 5. In FIG.5, the abscissa is the pore size diameter and the ordinate is the logdifferential intrusion. As shown in FIG. 5, the separators of Examples 1to 4 each have a first peak in a pore diameter range of 0.2 μm to lessthan 2 μm and a second peak in a pore diameter range of 2 μm to 30 μm.On the contrary, the separator of Comparative Example 1 has no secondpeak in the pore diameter distribution.

The lithium ion absorption potential of the negative electrode activematerial was measured by the method explained below.

The negative electrode was cut into a size of 1 cm×1 cm as a workingelectrode. The working electrode was made to face a counter electrodemade of a 2 cm×2 cm lithium metal foil with a glass filter separatorinterposed therebetween and a lithium metal was inserted as a referenceelectrode in such a manner that it was not to be in contact with boththe working electrode and the counter electrode. These electrodes wereplaced in a three-pole glass cell and the working electrode, counterelectrode and reference electrode were respectively connected to aterminal of the glass cell. EC and BL were mixed in a ratio by volume of25:75 and 2 mol/l of LiBF₄ was dissolved in the obtained mixture solventto prepare an electrolytic solution. 50 mL of this electrolytic solutionwas poured into the glass cell to allow the separator and electrode tobe sufficiently impregnated with the electrolytic solution and then, theglass cell was closed. The manufactured glass cell was placed in a 25°C. constant temperature bath to measure the lithium ion absorptionpotential of the working electrode when charged under a current densityof 0.1 mA/cm². The intermediate potential of the measured absorptionpotentials was found as the lithium ion absorption potential.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A secondary battery comprising: a positive electrode; a negativeelectrode containing a metal compound having a lithium ion absorptionpotential of 0.2V (vs.Li/Li⁺) or more; a separator which is providedbetween the positive electrode and the negative electrode, comprisescellulose fibers and pores having a specific surface area of 5 to 15m²/g, and has a porosity of 55 to 80% and a pore diameter distributionhaving a first peak in a pore diameter range of 0.2 μm (inclusive) to 2μm (exclusive) and a second peak in a pore diameter range of 2 to 30 μm;and a nonaqueous electrolyte.
 2. The secondary battery according toclaim 1, wherein the positive electrode contains at least one selectedfrom the group of a manganese composite oxide having a spinel structureand a metal-phosphorous oxide having an olivine structure.
 3. Thesecondary battery according to claim 2, wherein the metal-phosphorousoxide is represented by Li_(a)FePO₄ (0≦a≦1.1).
 4. The secondary batteryaccording to claim 1, wherein the metal compound is Li_(x)TiO₂ (0≦x≦1)or a lithium-titanium oxide having a spinel or rhamsdelite structure. 5.The secondary battery according to claim 1, wherein the specific surfacearea is 10 to 14 m²/g.
 6. The secondary battery according to claim 1,wherein the first peak is present at a pore diameter range of 0.3 μm(inclusive) to 2 μm (exclusive) and the second peak is present at a porediameter of 3 to 20 μm.
 7. The secondary battery according to claim 1,wherein the porosity is 62 to 80%.
 8. The secondary battery according toclaim 1, wherein a ratio of the cellulose fibers in the separator is 10to 100% by weight.
 9. A battery pack comprises a secondary battery, thesecondary battery comprising: a positive electrode; a negative electrodecontaining a metal compound having a lithium ion absorption potential of0.2V (vs.Li/Li⁺) or more; a separator which is provided between thepositive electrode and the negative electrode, comprises cellulosefibers and pores having a specific surface area of 5 to 15 m²/g, and hasa porosity of 55 to 80% and a pore diameter distribution having a firstpeak in a pore diameter range of 0.2 μm (inclusive) to 2 μm (exclusive)and a second peak in a pore diameter range of 2 to 30 μm; and anonaqueous electrolyte.
 10. The battery pack according to claim 9,wherein the positive electrode contains at least one selected from amanganese composite oxide having a spinel structure and ametal-phosphorous oxide having an olivine structure.
 11. The batterypack according to claim 10, wherein the metal-phosphorous oxide isrepresented by Li_(a)FePO₄ (0≦a≦1.1).
 12. The battery pack according toclaim 9, wherein the metal compound is Li_(x)TiO₂ (0≦x≦1) or alithium-titanium oxide having a spinel or rhamsdelite structure.
 13. Thebattery pack according to claim 9, wherein the specific surface area is10 to 14 m²/g.
 14. The battery pack according to claim 9, wherein thefirst peak is present at a pore diameter range of 0.3 μm (inclusive) to2 μm (exclusive) and the second peak is present at a pore diameter of 3to 20 μm.
 15. The secondary battery according to claim 9, wherein theporosity is 62 to 80%.
 16. The secondary battery according to claim 9,wherein a ratio of the cellulose fibers in the separator is 10 to 100%by weight.
 17. The secondary pack according to claim 9, wherein thesecondary battery is present in plural, the battery pack comprising abattery assembly comprising the plural batteries connected in series.18. A car comprising the secondary battery according to claim 1.